1. Field of the Invention
The present invention relates to a deterioration detecting apparatus for an exhaust gas sensor, the deterioration detecting apparatus diagnosing deterioration of the exhaust gas sensor, which is provided with a heater for heating a sensor element.
2. Description of Related Art
A recent vehicle is provided with an exhaust gas sensor (e.g., an air-fuel sensor, an oxygen sensor) on an exhaust gas pipe such that a feedback control of the air-fuel ratio is performed based on an output of the exhaust gas sensor in order to effectively purify exhaust gas by use of a catalyst. The exhaust gas sensor may not accurately sense the air-fuel ratio in a state where a temperature of the sensor element is equal to or less than an activation temperature. Therefore, the exhaust gas sensor includes the heater that heats the sensor element such that the temperature of the sensor element is increased to an activation temperature range by use of the heater.
Deterioration of the exhaust gas sensor may degrade of a degree of accuracy in detection performance of the exhaust gas sensor so that this may degrade efficiency in purifying the exhaust gas. Thus, some exhaust gas sensors are provided with a deterioration diagnosis function. For example, Japanese Unexamined Patent Publication No. H11-83791 (corresponding to U.S. Pat. No. 603,410) P.2 to P3 discloses a deterioration diagnosis method for the exhaust gas sensor, in which impedance (internal resistance) of the sensor element of the exhaust gas sensor is measured and the deterioration of the exhaust gas sensor is determined when the measured value of the impedance exceeds a target value (deterioration determining value). In Japanese Unexamined Patent Publication No. H11-83791, the target value (deterioration determining value) is set in consideration of that the impedance of the sensor element changes relative to the temperature of the sensor element. Specifically, the target value is set by estimating the impedance as well as the temperature of the sensor element based on power consumption and a temperature of the exhaust gas detected by a temperature sensor.
In the deterioration diagnosis method for the exhaust gas sensor in the above Japanese Unexamined Patent Publication No. H11-83791, it is determined only whether the deterioration is present or not based on a comparison between the measure value of the impedance of the sensor element and the target value (deterioration determining value). Thus, an actual progressing degree of the deterioration is disadvantageously not detected.
Also, in general, the impedance (temperature) of the sensor element changes correspondingly to external heat emission emitted by the sensor element in addition to heat absorption (an exhaust gas temperature, the power consumption of the heater) by the sensor element. However, in the deterioration diagnosis method in Japanese Unexamined Patent Publication No. H11-83791, the impedance (temperature) of the sensor element is estimated solely based on the heat absorption (the exhaust gas temperature, the power consumption of the heater). Therefore, a degree of accuracy in estimation may disadvantageously become low and also a degree of accuracy in diagnosing the deterioration may disadvantageously become low.